5 Experimental Fluid Mechanics
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two states of “no blowing” and “blowing” in the wind tunnel and the pressure distribution of the surface of the model is obtained by comparing the
computer with the calibration pressure data.
5.5 Flow Velocimetry Technique
1. Traditional measurement of time-average speed
For the time-average velocity of any point in the flow field, Pitot tube
velocimeter is often used. Pitot tube (as shown in Fig. 5.3) is a simple device
for measuring velocity based on Bernoulli equation of steady, incompressible,
and inviscid fluid. Since the test hole is facing the flow direction of the fluid, it
is not suitable for the flow of solid particles in the fluid. Pitot tube is a singlepoint contact speed measurement, which has a great impact on the measured
flow field, and the actual flow rate measurement has certain limitations. The
porous probe is a device for measuring the magnitude and direction of the
velocity, and mainly includes three-, five-, and seven-hole probes. Due to
the small range of flow angles measured by the three-hole probe, five- or
seven-hole probes are currently used. The five or seven-hole probe solves the
problem that the Pitot tube cannot measure the speed direction and can
measure the speed and pressure. The five-hole probe is inserted into the flow
field measuring point position, and the pressure measured by each hole is
appropriately corrected to obtain the velocity magnitude and direction of
the measured point of the flow field. When measuring the velocity with the
seven-hole probe, the pressure values of the seven holes are measured, and
the velocity, direction, total pressure, static pressure, and other parameters of
the measuring point can be obtained. The five or seven-hole probe has high
measurement accuracy, good reliability, simple structure, easy to damage the
probe, easy maintenance, and low cost. However, the five or seven-hole probe
is limited by its own measurement angle, and the measurement calibration
and calibration data processing are cumbersome.
2. Measurement of instantaneous speed
For the instantaneous velocity field, the velocity values include the timeaverage velocity and the pulsation velocity, and in particular, the velocity
pulsation with time is the basic data of the surface flow field characteristics. A commonly used measurement method in wind tunnels is a hot wire
361
two states of “no blowing” and “blowing” in the wind tunnel and the pressure distribution of the surface of the model is obtained by comparing the
computer with the calibration pressure data.
5.5 Flow Velocimetry Technique
1. Traditional measurement of time-average speed
For the time-average velocity of any point in the flow field, Pitot tube
velocimeter is often used. Pitot tube (as shown in Fig. 5.3) is a simple device
for measuring velocity based on Bernoulli equation of steady, incompressible,
and inviscid fluid. Since the test hole is facing the flow direction of the fluid, it
is not suitable for the flow of solid particles in the fluid. Pitot tube is a singlepoint contact speed measurement, which has a great impact on the measured
flow field, and the actual flow rate measurement has certain limitations. The
porous probe is a device for measuring the magnitude and direction of the
velocity, and mainly includes three-, five-, and seven-hole probes. Due to
the small range of flow angles measured by the three-hole probe, five- or
seven-hole probes are currently used. The five or seven-hole probe solves the
problem that the Pitot tube cannot measure the speed direction and can
measure the speed and pressure. The five-hole probe is inserted into the flow
field measuring point position, and the pressure measured by each hole is
appropriately corrected to obtain the velocity magnitude and direction of
the measured point of the flow field. When measuring the velocity with the
seven-hole probe, the pressure values of the seven holes are measured, and
the velocity, direction, total pressure, static pressure, and other parameters of
the measuring point can be obtained. The five or seven-hole probe has high
measurement accuracy, good reliability, simple structure, easy to damage the
probe, easy maintenance, and low cost. However, the five or seven-hole probe
is limited by its own measurement angle, and the measurement calibration
and calibration data processing are cumbersome.
2. Measurement of instantaneous speed
For the instantaneous velocity field, the velocity values include the timeaverage velocity and the pulsation velocity, and in particular, the velocity
pulsation with time is the basic data of the surface flow field characteristics. A commonly used measurement method in wind tunnels is a hot wire